Vehicles which have a hydrostatic drive are generally exposed to extreme fluctuations with regard to the load to be handled and with regard to the vehicle speed to be realized. Frequent and rapid braking of the vehicle is therefore necessary. In particular, the wear of the conventional brakes is therefore relatively high. In the case of vehicles having hydrostatic drives, the vehicle is often braked by an infinitely variable change in the transmission ratio. However, the rotational speed of the internal combustion engine providing the requisite drive power is increased in order to absorb the kinetic energy of the vehicle during the braking operation. This increase in the rotational speed is detrimental to the service life of the internal combustion engine and in addition leads to generally unacceptable high noise emissions.
Hydrostatically driven fork stackers are typically driven only by means of a travel-direction switch and an accelerator pedal, which controls the internal combustion engine and the respective transmission ratio. This enables the driver to operate the vehicle in a simple manner with regard to the driving and therefore enables the driver to fully concentrate on the operations to be carried out by the vehicle. Fork stackers usually have no conventional service brake and utilize the hydrostatic traction drive as a brake. Thus, all the kinetic energy of the vehicle, during braking or deceleration of the latter, is directed via the hydrostatic transmission to the internal combustion engine, which typically is a diesel engine. Since the internal combustion engine only has limited power absorption, a large proportion of the vehicle energy is transmitted into the flywheel, so that the rotational speed increases considerably. The problem described occurs in relation to the vehicle speed, its mass and the requisite braking deceleration. It has therefore become known to provide a retarder device, by means of which additional energy can be converted into heat.
In wheeled loaders and off-highway stackers, because of considerably higher travel speeds to be achieved, conventional service brakes are also provided. In addition to the conventional service brakes, a retarder device, which works in parallel with the service brake, is provided in these vehicles. The retarder device serves in principle to reduce the brake wear and enables the vehicle to travel a longer distance on a gradient without reaching the critical temperature of the service brake.
A feature common to the existing systems during the function of the retarder is a purely mechanical drive branch, so that the kinetic energy of the vehicle can pass to the variable displacement pump.
Therefore, the principal object of this invention is to provide a vehicle having a permanent hydrostatic traction drive or a hydraulic control apparatus wherein a retarder valve device provided instead of or in addition to a brake device, works essentially free of wear; the internal combustion engine is not overloaded; and the number of components for this arrangement is reduced.
These and other objects will be apparent to those skilled in the art.